Hong Kong police were carrying out a major bomb disposal operation on Saturday afternoon involving the evacuation of 1,300 people after the discovery of a 450kg (1,000 lbs) wartime explosive at a Wan Chai construction site.

Senior bomb disposal officer Tony Chow Shek-kin said the ANM-65 device was a remnant from the second world war and had been dropped from the air by American bombers sometime between 1941 and 1945.

It contained about 225kg of explosives, which police expected to have defused by around midnight.

“The front detonator is damaged and would pose a danger if we were to move the bomb. So we have to dismantle it at the scene,” Chow said.

“It could cause extensive damage. If the bomb explodes the force could affect the surrounding area within 200 metres, with fragments flying as far as 2,000 metres.”

The bomb was discovered at a building site for the Sha Tin-Central rail link, on Harbour Road and Tonnochy Road. A construction worker made a report to police at 7.40am on Saturday.

The large cylindrical device, 140cm long and 45cm in diameter, was unearthed 15 metres down in the soil.

Chow said his team would have to cut two holes in the bomb casing and burn the explosives inside in a process that would take about three hours. The area within a 400-metre radius was cleared while officers spent more than five hours on Saturday afternoon protecting the site using sandbags. The team planned to take any remaining explosives after the operation back to their base for further dismantling.

Maxim Kwok Mei-sum, police divisional commander for Wan Chai, said the force had evacuated about 1,300 people, mainly from the area facing the construction site.

“These included residents and workers from offices, the Hong Kong Convention and Exhibition Centre and nearby hotels,” Kwok said. “Residents can be sheltered at the Wan Chai Activities Centre for the time being.”

The northbound section of Tonnochy Road near the sports centre was closed to traffic, and other roads to north Wan Chai were to be closed after midnight, including small sections of Convention Avenue and Harbour Road, police said.